As art for purifying exhaust from a gasoline engine or diesel engine or other internal combustion engine, it is known to use a NOX purifying catalyst. However, in such a NOX purifying catalyst, there is a problem in that if the metal included as the catalyst ingredient is exposed to in particular an atmosphere having high oxygen concentration, the surface of the metal is covered by oxygen, i.e., the surface of the metal suffers from so-called oxygen poisoning, resulting in reduced NOX purifying performance of the NOX purifying catalyst.
On the other hand, a platinum group element such as rhodium (Rh) which is generally used as a catalyst ingredient in a NOX purifying catalyst is being used in increasingly larger amounts along with the toughening of automobile emission controls. For this reason, depletion of resources is becoming a concern. Therefore, it has been considered necessary to reduce the amount of use of the platinum group element and, in the future, to replace the platinum group element in role with another metal, etc.
Japanese Patent Publication No. 2001-003733A describes an exhaust gas purification system in a diesel engine which treats exhaust gas by a NOX catalyst containing an active metal wherein when the exhaust gas is in a lean state, the NOX contained in the exhaust gas is made to react with the active metal in the catalyst to thereby break it down to nitrogen and oxygen and, when this state progresses and the catalyst activity becomes unsuitably low or is learned to have become low, a rich state of the exhaust gas is formed to thereby remove an oxide film of the NOX catalyst, or the oxygen ingredient and/or sulfur ingredient which is sometimes present accompanying it, to thereby regenerate the NOX catalyst and restore the NOX breakdown activity, then the NOX breakdown stage under the above lean state is switched to, the NOX breakdown stage and catalyst regeneration stage being combined by a suitable time pattern to enable exhaust gas to be treated over a long time at a high efficiency. Furthermore, Japanese Patent Publication No. 2001-003733A specifically discloses a configuration comprising copper (Cu) supported on a metal oxide support as the above NOX catalyst containing an active metal.
Further, Japanese Patent Publication No. 2006-022754A, Japanese Patent Publication No. 2002-303176A, Japanese Patent Publication No. 5-133260A, etc., also describe an exhaust gas purification system or catalyst regeneration method which switches the air-fuel ratio of the exhaust gas, which flows into the catalyst, to lean or rich air-fuel ratio to thereby regenerate or improve the activity of the catalyst.
Japanese Patent Application No. 2011-183964 describes an exhaust purification system for an internal combustion engine comprising a basic structure consisting of a first stage base metal catalyst which mainly partially oxidizes HC to generate CO and a second stage base metal catalyst which purifies NOX by reduction, and describes, as a preferable embodiment, an embodiment comprising making exhaust gas having an air-fuel ratio slightly fuel richer than a theoretical air-fuel ratio flow into the first stage base metal catalyst; providing a third stage base metal catalyst for purifying HC and CO by oxidation at the downstream side of the basic structure; and introducing air into the exhaust gas flowing into the third stage base metal catalyst by an air introducing means provided between the basic structure and the third stage base metal catalyst. Japanese Patent Application No. 2011-183964 describes that copper (Cu) can be used as a catalyst ingredient in the second stage base metal catalyst which purifies NOX by reduction. Further, Japanese Patent Application No. 2011-183964 describes that according to the exhaust purification system comprising the above configuration, purification of NOX by reduction is promoted by a CO—NO reaction in the second stage base metal catalyst, and CO and HC remaining in the exhaust gas are purified by oxidation in the third stage base metal catalyst.
Further, it is known that an oxygen sensor, air-fuel ratio sensor, etc., are generally used in order to control the air-fuel ratio of an exhaust gas flowing into a catalyst, and feedback control is performed on the basis of output values from these sensors so that the air-fuel ratio becomes a specific target value (for example, see Japanese Patent Publication No. 2008-298044A, Japanese Patent Publication No. 2007-077997A, Japanese Patent Publication No. 2004-278427A, Japanese Patent Publication No. 8-270479A, etc.).
Further, Japanese Patent Application No. 2011-274544 describes an exhaust gas purification system using a base metal-based NOX purifying catalyst comprising a base metal supported on a support, in which a target value of an air-fuel ratio is set on the basis of an intake air amount (Ga) of an internal combustion engine, and the amount of a fuel injected into the internal combustion engine, etc., is adjusted so that the air-fuel ratio becomes the set value.